The present invention broadly relates to trivalent chromium electrolytes, and more particularly to an improved trivalent chromium electrolyte composition which is substantially more tolerant to the presence of deleterious contaminating metal ions such as nickel, zinc, iron, copper and lead which accumulate and progressively increase in concentration during normal commercial operation of the bath. When one or more of such metal ion impurities attain concentration levels in which they are present in only relatively trace quantities, the chromium electrodeposit is adversely affected by the presence of black streaks, clouds and hazes which are sometimes further accompanied by a loss or reduction in covering power rendering such chromium electrodeposits commercially unsatisfactory.
In recognition of the problem associated with extraneous metal ion contamination of such trivalent chromium baths, it has heretofore been proposed in accordance with U.S. Pat. No. 4,038,160 to add small amounts of water soluble ferrocyanide compounds to the plating solution to effect a precipitation of such contaminating metal ions which thereafter are removed by filtration. While such proposed ferrocyanide treatment has been found effective in many instances, the treatment is costly and time consuming and the ferrocyanide precipitating agent itself can adversely affect the performance of the trivalent chromium electrolyte when employed in amounts that leave a residual excess of the precipitating agent dissolved in the bath. This necessitates further treatment by the intentional addition of contaminating metals to effect a precipitation of the excess precipitating agent present.
It has also been proposed to remove such contaminating metal ions through an electrolytic purification technique by which the bath is electrolyzed over a period of time employing a cathode on which a codeposition of the contaminating metal ions is effected. Unfortunately, while such an electrolytic purification technique is somewhat effective for reducing copper ion contamination, it is relatively ineffective for removing nickel and zinc ions and is only partially effective for removing iron.
The present invention provides an improvement over such prior art techniques by providing a trivalent chromium electrolyte which is more tolerant to the presence of one or more of such contaminating metal ions masking or hiding their deleterious effects thereby providing for a longer useful operating life of the bath under normal commercial operating conditions. Additionally, the present invention enhances the codeposition of such contaminating metal ions thereby substantially reducing the rate of buildup of concentration of such contaminating ions during the normal commercial operation of the bath which in those instances in which the rate of contamination is relatively low, is adequate in and of itself to prevent accumulation of such metal ions to levels at which deleterious results are obtained. The present invention further contemplates a method for rejuvenating or restoring the performance of a trivalent chromium electrolyte which has been detrimentally affected by the accumulation of such contaminating metal ions whereby the concentration thereof is reduced restoring the electrolyte to commercially satisfactory operating conditions.